1. Field of the Invention
This invention relates to a liquid pressure extrusion method and device for extruding a billet into a tube or tubular member, in which combined dies, such as of a port-hole, bridge and spider type, are used.
2. Description of the Prior Art
Included in extrusion methods for extruding a tube or tubular member under a liquid pressure are (1) a mandrel fixed type, in which a mandrel is fixed relative to a hollow billet, and (2) a movable mandrel type, in which a mandrel is movable relative to a hollow billet.
The former finds a wide use in industries, because the mandrel serving as a reference core is stationary, presenting high accuracy for extrusions. However, this method suffers from a shortcoming in that the mandrel requires a support therefor, i.e., a sleeve, so that the stroke of a pressing ram is shortened to an extent corresponding to the length of the sleeve. This in turn leads to a decrease in the length of a billet which is extrudable, thus providing an extrusion of an amount half that of an extrusion obtained from a solid billet.
On the other hand, the latter method provides an advantage in that, unlike the former case, this dispenses with a mandrel support (sleeve), with the resulting freedom of the limitation of a length of a stroke of a pressing ram, thus enabling extrusion of a hollow billet into a tube of a desired length as in the case of a solid billet extrusion. However, this latter method poses a problem that the rear end face of a billet tends to move due to a pressure acting on the flange portion of a mandrel, on which the billet abuts, thus failing to maintain a consistent condition for the moving billet, in contrast to the case where the outer surface of a billet as a whole is directly subjected to a liquid pressure. As a result, this results in a slight inbalance or failure in balance between the inner and outer pressures acting on the extruding portion of dies, presenting difficulties in maintaining dimensional accuracy of an extrusion constant. In addition, after the completion of an extrusion, the mandrel remains pressed deep into the end of an extruded product, so that many man hours are required for withdrawing the mandrel from the end thereof. This apparently lowers the efficiency of operation as well as poses a difficulty to be solved by technology.